1. Field of the Invention
The present invention relates to a radio apparatus which performs an HARQ-based automatic retransmission.
2. Description of the Related Art
The Third-Generation Partnership Project (3GPP) defines specifications for high speed downlink packet access (HSDPA) in order to enable high speed data transfers.
One feature of HSDPA is automatic retransmission using hybrid ARQ (HARQ: Hybrid Automatic Repeat Request). According to HARQ, a terminal performs error detection on data from a radio base station, and requests a retransmission from the radio base station if an error is detected. Based on the nature of the response from the terminal, the radio base station determines whether a retransmission is necessary and, if necessary, retransmits the data.
FIG. 1 is a diagram illustrating the flow of signal processing on the transmission side of a physical layer by a conventional general radio base station.
Referring to FIG. 1, the radio base station performs, on data to be transmitted, CRC attachment processing 81, bit scrambling processing 82, code block segmentation processing 83, and channel coding processing 84, and then performs HARQ (Physical Layer Hybrid-ARQ functionality) processing 85. HARQ processing 85 is the processing for automatic retransmission using HARQ.
After this processing, the radio base station performs physical channel segmentation processing 86, HS-DSCH interleaving processing 87, constellation re-arrangement processing 88, and physical channel mapping processing 89, and transmits data to a terminal on a physical channel (phCH).
FIG. 2 is a block diagram illustrating the configuration of an HARQ processing unit which performs HARQ processing in the signal processing of the physical layer illustrated in FIG. 1. Referring to FIG. 2, HARQ processing unit 90 has bit separation unit 91, first rate matching unit 92, virtual IR buffer 93, second rate matching unit 94, and bit collection unit 95. Then, HARQ processing unit 90 performs rate matching on input data c twice, before and after the virtual buffer, using parameters s, r of redundancy version (hereinafter called the “RV parameters”) that are applied, for example, from a scheduler (not shown) in the MAC layer.
Virtual IR buffer 93 is a virtual buffer which performs buffering with a predefined buffer size.
Bit separation unit 91 separates input data c from channel coding processing 83 into systematic bits, parity 1 bits, and parity 2 bits.
First rate matching unit 92 performs rate matching processing on the systematic bits, parity 1 bits, and parity 2 bits that are from bit separation unit 91, and sends the resulting respective bits to virtual IR buffer 93. In this event, first rate matching unit 92 actually performs the processing only on the parity 1 bits and parity 2 bits (RM_P1_1, RM_P2_1), but passes the systematic bits, as they are, therethrough.
Second rate matching unit 94 performs the rate matching processing on data from virtual IR buffer 93, and sends the resulting data to bit collection unit 95. In this event, second rate matching unit 94 performs the processing on the systematic bits, parity 1 bits, and parity 2 bits (RM_S, RM_P1_2, RM_P2_2). In the rate matching in second rate matching unit 94, the number of bits Ndata of output data w is used as a parameter in addition to the RV parameters.
Bit collection unit 95 interleaves the systematic bits, parity 1 bits, and parity 2 bits from second rate matching unit 94, and delivers them as output data w.
Ndata used in second rate matching unit 94 can be derived from the number of codes and a modulation scheme after the scheduler decides to sent the data to a user who has been selected to receive it. Specifically, Ndata can be calculated by Equation (1):Ndata=3×(Number of Bits of Modulation Scheme)×(Number of Codes of HS-PDSCH)  (1)
In Equation (1), the number of bits of the modulation scheme is 320 bits for QPSK, and 640 bits for 16QAM.
As described above, the automatic retransmission is performed using the HARQ processing in HSDPA, but it is important to appropriately select the redundancy version in the retransmission in accordance with a change in the environment of a propagation path in order to improve the throughput of the system. A variety of proposals have been made so far for methods to select the redundancy version (for example, see JP-A-2004-112597).
However, no method has been established at present for selecting an appropriate redundancy version, and standardization has not been achieved. Unless an appropriate redundancy version is selected whenever retransmission occurs, an appropriate resource will not be allocated to appropriate bits, resulting in an increase in the number of times of retransmission.
For example, in the retransmission, the number of bits (Nsys) of the systematic bits applied to HARQ unit 90 is the same as that in the first transmission. If there is a significant deterioration in the propagation path environment (reduction in CQI (Channel Quality Indicator) value) in the retransmission, as compared with the first transmission, and if limits are imposed on the number of available codes, only part of the systematic bits, rather than all of them, can be retransmitted depending on the selected redundancy version.
Also, even if retransmissions are repeated from the radio base station with a redundancy version which handles the systematic bits as nonpriority in a state where the systematic bits have not yet been correctly received by a terminal, correct decoding is not accomplished in the terminal, resulting in repeated retransmissions.
Since appropriate resources are not allocated in the HARQ-based automatic retransmission due to the failure to establish a method of selecting a redundancy version in transmission, it is difficult to improve the data throughput.